Motors are now used for various purposes. For example, in an information recording/reproducing apparatus such as a VTR, or a magneto-optical disk drive, a motor is used for driving a magnetic head or an information recording medium.
In an information recording/reproducing apparatus of this type, a change in a relative speed of the magnetic head with respect to the information recording medium likely causes deterioration of an image quality or a sound quality, and causes noises. Therefore, it is necessary that the motor used therein should be controlled so that changes in the rotating speed of the motor are restrained. To achieve this purpose, a motor control device described below has been proposed.
FIG. 6 is a block diagram of a usual motor control device. As illustrated in this figure, a characteristic of a motor 31 is represented as a transfer function of 1/ (Js+C), where J represents a moment of inertia of the motor 31, C represents a coefficient of viscosity which is determined with an influence of a back electromotive force on the motor 31 taken into consideration, and s represents the Laplace operator.
An output .omega. of this motor control device is a velocity signal indicating a rotating speed of the motor 31, which is detected by a velocity detector (not shown). Note that here it is premised that a velocity signal indicating an accurate rotating speed of the motor 31 can be obtained by the velocity detector.
A comparator 32 is provided on an input side of the motor control device. The comparator 32 outputs an error signal E.sub.rr as a difference between a velocity reference value .omega..sub.ref as a reference speed of the motor 31 and the velocity signal .omega..
A driving circuit 33 is connected to an output terminal of the comparator 32 so that the error signal E.sub.rr is supplied to the driving circuit 33. The driving circuit 33 conducts a control operation which is a combination of proportional compensation and integral compensation, and outputs a driving voltage V.sub.r. Note that the proportional compensation relates to speed control whereas the integral compensation relates to phase control. A characteristic of the driving circuit 33 is represented as a transfer function of K.sub.p +K.sub.i /s, where K.sub.p represents a proportional gain whereas K.sub.i represents an integral gain.
A block 34 is connected to an output terminal of the driving circuit 33. The block 34 converts the driving voltage V.sub.r into a driving torque T.sub.m. A characteristic of the block 34 is represented as a coefficient K.sub.t, and the coefficient K.sub.t is called the torque constant. Note that the torque constant is a constant found by calculating (driving torque)/(driving voltage).
A disturbance torque T.sub.g is applied to the motor 31, and a subtracter 35 is equivalently connected to an output terminal of the block 34 so that the driving torque T.sub.m and the disturbance torque T.sub.g applied to the motor 31 are supplied to the subtracter 35. Note that the subtracter 35 is shown in the figure for convenience sake so as to indicate that the disturbance torque T.sub.g is applied to the motor 31, and such a circuit as the subtracter 35 does not actually exist in this arrangement. The subtracter 35 subtracts the disturbance torque T.sub.g from the driving torque T.sub.m, and sends a net driving torque T thus obtained to the motor 31. The motor 31 rotates in accordance with the net driving torque T, and sequentially the velocity signal .omega. is detected.
The velocity signal .omega. is sent to the comparator 32 through a negative feedback loop. Thus, the control operation is carried out so that the velocity signal .omega. coincides with the velocity reference value .omega..sub.ref. Note that the negative feedback loop which feeds back the velocity signal .omega. is hereinafter referred to as loop A. An open loop transfer function G.sub.open of this control system is given as: EQU G.sub.open =(K.sub.p +K.sub.i /s).multidot.K.sub.t .multidot.{1/(J.multidot.s+C)} (1)
FIG. 3 is at graph illustrating an example of a gain characteristic of the open loop transfer function G.sub.open, which is indicated by a solid line in the graph. A frequency f.sub.c indicated on a horizontal axis of the graph is a gain crossover frequency. A response G.sub.comp of the motor 31's rotating speed to the disturbance torque T.sub.g in this control system is given as: EQU G.sub.comp ={1/(J.multidot.s+C)}/(1+G.sub.open) (2)
The response G.sub.comp has a chevron characteristic, as indicated by a broken line in the graph of FIG. 2. This characteristic is understandable, considering that the following relational expressions can be obtained using the above equation (2): ##EQU1##
In other words, a disturbance suppressing characteristic on a side of a low band stems from an effect of the feedback control which is expressed by the above equation (3), while a disturbance suppressing characteristic on a side of a high band stems from an effect of the moment of inertia of the motor 31 which is expressed by the above equation (4).
However, further advanced motor control devices are recently demanded, and the above-described disturbance suppressing characteristic obtained by the feedback control is insufficient. For example, in VTR apparatuses, jitters of low frequencies cause edge noises due to signal cross talks on magnetic tapes, or cause track curving in the case of recording with respect to narrow tracks, or the like. Therefore, in the case where a rotating speed of a capstan motor is to be controlled, to enhance efficiency in suppressing disturbances in the low and middle bands is a matter of great importance.
As a method for obtaining a sufficient efficiency in suppressing disturbances particularly in the low and middle bands, a control method using a disturbance observer is well known (for example, see the Japanese Publication for Unexamined Patent Application No. 3-155383/1991 (Tokukaihei 3-155383)). According to this method, the disturbance suppressing efficiency is enhanced by estimating the disturbance torque applied to the motor and adding the disturbance torque thus estimated to the driving voltage of the motor in accordance with a feedforward control method.
FIG. 7 is Et block diagram illustrating a motor control device using a minimum-dimensional observer disclosed in the Japanese Publication for Unexamined Patent Application 3-155383/1991. In FIG. 7, the members having the same structure (function) as those in FIG. 6 will be designated by the same reference numerals and their description will be omitted. Note that in a transfer function of the motor 31, a term of viscosity is omitted.
The motor control device of FIG. 7 differs from that of FIG. 6 in that a disturbance estimating unit 40, a disturbance compensation gain 50 (coefficient: 1/K.sub.tn), and an adder 36 are added. In FIG. 7, J.sub.n represents a nominal value of the moment of inertia of the motor 31, K.sub.tn represents a nominal value of the torque constant K.sub.t, and g represents a positive constant indicating a band for the disturbance observer 40.
The disturbance estimating unit 40, supplied with a motor driving voltage V and a velocity signal .omega., outputs a signal T.sub.g1. To be more specific, the motor driving voltage V is supplied to a block 41 so as to be multiplied by a coefficient g.multidot.K.sub.tn. The velocity signal .omega. is supplied to a block 42 so as to be multiplied by the coefficient g.sup.2 .multidot.J.sub.n, while it is supplied to a block 43 so as to be multiplied by a coefficient g.multidot.J.sub.n. Then, outputs of the blocks 41 and 42 are added by an adder 44, and the added result is supplied to a first-order lag element represented as a block 45. Sequentially, an output of the block 43 is subtracted from an output of the block 45 by a subtracter 46, thereby resulting in that the signal T.sub.g1 is outputted.
The signal T.sub.g1 thus outputted from the disturbance estimating unit 40 is supplied to the disturbance compensation gain 50, so as to be multiplied by the coefficient 1/K.sub.tn. As a result, a disturbance compensation signal VF is outputted from the disturbance compensation gain 50. The disturbance compensation signal VF is added by the adder 36 to a driving voltage V.sub.r which is outputted by the driving circuit 33, the added result being a motor driving voltage V. The motor driving voltage V is supplied to a block 34, and then, it is converted to a net driving torque which is supplied to the motor 31, as described above.
The disturbance compensation signal VF is given as: ##EQU2##
According to the equation (5), FIG. 7 can be equivalently re-drawn into FIG. 8. A transfer function indicating a characteristic of the block 47 in FIG. 8 is obtained by modeling a reverse characteristic of the motor 31, and the transfer function is hereinafter referred to as motor reverse model.
An output T.sub.g of the subtracter 48 in FIG. 8 can be given as: ##EQU3## Given that K.sub.tn =K.sub.t and J.sub.n =J, the equation (6) is reorganized as follows: EQU T.sub.g =T.sub.g (7)
As a result, an estimated value of the disturbance torque T.sub.g can be expressed using the output T.sub.g. Therefore, the output T.sub.g is hereinafter referred to as disturbance estimation signal.
In FIG. 8, a block 49 whose transfer function is given as g/(s+g) is a first-order low-pass filter having a cut-off frequency of g/(2.pi.)[Hz]. Therefore, an output T.sub.g of the block 49 is a low band component of the disturbance estimation signal T.sub.g.
In short, it is found that the control system (see FIG. 7) using the minimum-dimensional observer essentially:
(1) estimates the disturbance torque T.sub.g ;
(2) adds the low band component of the disturbance torque thus estimated to the driving voltage V in accordance with the feedforward control method; and
(3) by doing so, cancels the low band component of the disturbance torque T.sub.g.
However, with the above arrangement, a lot of calculations are necessary to obtain the disturbance compensation signal VF. To realize this system in hardware, elements in the motor control device are caused to increase, thereby making it difficult to miniaturize the device, while adding to costs. To realize this system in software, since a long period of time is required for carrying out a lot of calculations, arises a problem that performance of the device deteriorates due to the insufficient period for calculations and delay in response.
To avoid the delay in response, expensive microcomputers capable of high-speed processing are needed, and this also leads to a problem of adding to the costs of the device.
Furthermore, since nominal values of the motor characteristics are used in the foregoing arrangement when calculations are carried out, desired control performance of the motor control device cannot be achieved in the case where real values and nominal values are different due to, for example, irregularity in the motor characteristics.